The objective is to characterize the protective antigens and toxic products of Coxiella burnetii. Structure and functional aspects of the immunomodulation by surface components of C. burnetii are being analyzed in vivo and in vitro as candidates for subunit vaccines. We have shown that C. burnetii absorbs normal serum acute phase proteins to its surface which may enhance opsonization by phagocytic cells; surface polysaccharide is implicated as a virulence factor of phase I cells whereas phase II, avirulent cells have lost this surface component; mouse monoclonal antibodies directed to surface polysaccharide (IgG2a) shows specificity for phase I cells whereas monoclonal antibodies (IgG2b) directed to surface protein shows specificity for phase II cells; an enzyme-linked immunoabsorbent assay has been developed for the detection of early antibody (phase II immunogen) and late antibody (phase I immunogen); subfractionation of C. burnetii cells into peptidogylcan (PG) insoluble fractions have shown that the phase II immunogen is a protein covalently bound to the PG whereas the phase I immunogen is a soluble polysaccharide; and phase conversion (I to II) in C. burnetii is similar to the smooth to rough variation described for gram-negative bacteria.